Prac #7: Constructing a Food Web Name:___________________ Introduction To study an ecosystem, we begin by determining which organisms are present. However, understanding the ecosystem requires us to do more than just compile a list of the plants and animals it contains. To understand how an ecosystem functions, we need to know how the organisms of the ecosystem interact with each other. Most interactions between species come down to an understanding of ‘who eats whom’. In order to answer this question, we need to observe some feeding relationships. This is not always easy, as many interactions are difficult to see; however, careful observation allows us to deduce the movement of energy and nutrients from organism to organism through an ecosystem. Energy and nutrients enter most ecosystems when plants capture light energy from the sun and, by the process of photosynthesis, convert carbon dioxide and water into sugar. Photosynthetic organisms are called producers. All other organisms consume their food ready-made, so they are called consumers. Consumers include: Herbivores – consumers that feed directly on producers. Carnivores – consumers that eat other consumers (Carnivores include first-order carnivores, which feed directly on herbivores, and second-order carnivores, which feed directly on first-order carnivores. Scavengers and detritivores – consumers that eat dead plants and animals and their waste products. Decomposers – consumers that break down dead material. Bacteria and fungi are decomposers. The feeding relationship between organisms can be summarized in a food chain. Since one organism will often have more than one food source, food chains will often interlink and form a food web. A food web describes the feeding interactions of the community. Purpose 1. To use a series of observations of feeding relationships to construct food webs for two communities found in a suburban garden. 2. To make some comparisons between, and draw some inferences about, the feeding relationships in the two communities. Part A: Around a Grapevine These observations were made at a grapevine in a suburban garden. Organism Grapevine Vine moth Yeast (fungi) Vinegar fly Blackbird Earthworm Snail Silver-eye Black house-spider Bee Observation Deciduous vine, flowers in spring, fruit (grapes) ripens in autumn Larva (caterpillar) feeds on vine leaves Grows in juice of damaged grapes Feeds on yeasts Feeds on grapes and other soft fruits, vine moth larvae, earthworms, spiders and snails; permanent resident of garden Feeds on decaying leaves Feeds on decaying leaves Migratory bird which feeds on insects and fruits Feeds on adult vine moths Feeds on nectar and pollen Procedure 1. Start by writing ‘grapevine’ at the bottom of a blank piece of paper. Write the names of, and draw arrows to, those organisms that use the grapevine as a food source. The ‘grapevine’ includes leaves on the vine, leaves on the ground, and grapes. 2. Add the names of the organisms that feed on those that use the grapevine as a food source. Draw in arrows to indicate the feeding relationships. Make sure the arrowhead always points towards the organism doing the eating. In this way it is possible to trace the flow of energy and nutrients (matter) from one organism (the one being eaten) to the next (the one doing the eating). 3. Finally, check through the data and make sure you have not left out any observations about feeding relationships. At this point, you may wish to redraw your food web in a clearer form. 4. From the food web: a. Name two herbivores b. Name an organism that acts as a detritivore c. Write down a food chain in which the blackbird acts as: i. A herbivore ii. A first-order carnivore iii. A second-order carnivore d. Why type of consumer is yeast in this food web? Explain. Part B: Around a Eucalypt In a part of the garden nearby, eucalypts and other native species were growing. The following observations were made in this area. Organism Eucalypts Cup moth Sawfly Bee Wattlebird Silver-eye Orb web spider Beetle Earthworm Observation Various species that flower from spring to autumn; flowers produce large amounts of pollen Larvae feed on eucalypt leaves Larvae feed on eucalypt leaves Feed on nectar and pollen Feed on nectar, pollen, fruits (including grapes) and insects Migratory bird which feeds on insects and fruits Feeds on flying insects Feeds on eucalypts flowers Feeds on decaying leaves Procedure 5. Using the method from part A, construct a food web for this community Discussion The two sets of data suggest that some organisms move between the two communities in search of food. This links the two food webs. 6. From the data, explain which organisms provide this link 7. It has been observed that insect species such as vine moths, vinegar flies and cup moths disappear from the food web during winter. Suggest why this occurs and how they might survive to reappear each spring. 8. While silver-eyes appear during the fruit season but leave afterwards, only to reappear the next year, blackbirds remain as permanent residents. What feeding habits do blackbirds possess that enable them to survive all year in this community? 9. Earthworms are also year-round inhabitants of the two communities. Explain how earthworms are able to survive and remain active throughout the winter when many insects cannot. Conclusion Write a short paragraph detailing what you have learnt from this activity. Discussion Answers 6. Earthworm, bee, silver-eye 7. Move to a warmer climate to avoid the cold Could be the time of their life cycle where they lay eggs and die 8. Blackbirds eat earthworms, snails,. Vine moths, black house-spiders. Don’t rely on the fruit to survive, so when fruit season is over, they can stay. 9. Live underground, therefore warmer Feed on dead leaves from grapevine, so can still find food Sample Conclusion By completing this prac, we discovered that food webs are constantly changing due to animals leaving or arriving in the area. This may occur due to weather conditions, food availability, availability of animals to breed with, and other major changes. Not all food webs are closed webs because there is constant interaction between webs.